A treatment system includes a treatment tool and an energy source apparatus used to supply electrical energy to the treatment tool. The treatment tool includes a heater and bipolar electrodes to grip a treatment target. The energy source apparatus includes a processor to control the output to the bipolar electrodes and the heater, respectively. The processor directs the output to the bipolar electrodes and detects an initial value of the impedance of the treatment target. The processor determines whether or not the impedance has reached a minimum value and then retrieves a parameter that is detected before the minimum value of the impedance is detected. The processor performs a first process and/or a second process based on the acquired parameter after the minimum value is detected. The first process determines whether or not the output to the heater is required. The second process sets a target value.
